How Many Friends Does One Person Need? Dunbar’s Number and Other Evolutionary Quirks by Robin Dunbar

One of the many perils of being on Facebook is discovering that your friends have hugely more friends than you do. According to Robin Dunbar, we shouldn't feel so bad. Professor of evolutionary anthropology at Oxford, he points out that most of the people who parade ridiculous numbers of friends on Facebook (more than about 200, say) "invariably know little or nothing about most of the individuals on their list". You probably knew that already. What you might not know is that there is an evolutionary limit to the number of friends we can meaningfully have, whether in Facebook or in real life.

In 1993, Dunbar, a primatologist, put forward a remarkable little proposition that has since become known as Dunbar's Number, to which he devotes several chapters in this enjoyable collection of ­popular-science articles. Dunbar knew that one of the reasons that apes had evolved such big brains was